The present invention relates to the field of turbine machines and is aimed at a means for controlling the clearance there is between the tips of the moving turbine blades and the casing.
A gas turbine engine conventionally comprises a compressor, in one or more stages, a combustion chamber and one or more turbine stages. The compressor, which is connected to the turbine, supplies the combustion chamber with air and the hot gases produced are directed onto the turbine in order to extract their energy. The compressor and turbine rotors have sets of blades at their periphery moving at right angles to the engine axis inside annular stator components that form shroud rings with respect to which they enjoy an operating clearance. This clearance needs to be large enough that no friction will slow the rotation of the moving parts but needs to be controlled in order to prevent a substantial amount of fluid from being diverted away from the active surfaces of the sets of blades. In order to ensure the highest possible efficiency, it is therefore important to control this clearance.
The present invention is concerned with the operating clearance of a turbine motor and more especially of the rotor positioned immediately downstream of the combustion chamber. In a multiple-spool engine, that is to say an engine comprising two or more, generally no more than three, independent shafts, this will be the high-pressure spool.
The radial clearances at the blade tips are the result of the various radial thermomechanical movements between the rotors and the stators. FIG. 1 shows an axial half section of a gas turbine engine 1, viewed in the region of the high-pressure turbine. The turbine rotor 3 comprises a disk 31, provided with blades 33 distributed around its rim, and mounted transversely on a central shaft. The rotor is positioned downstream of a nozzle guide vane stage 5 communicating with the combustion chamber 7 only the bottom of which can be seen here. The casing 9 is made up of several shell rings assembled by flanges. There is a distinction between the combustion chamber casing 91 and the high-pressure turbine casing 93. The two casings are held by a flanged assembly 95. The casing supports the elements of the combustion chamber, the upstream 5 and downstream 15 nozzle guide vanes and a support 11 for a shroud ring 13.
The radial clearance between the tips of the blades 33 and the shroud ring 11 is thus the resultant of several types of movement:                thermal displacements resulting from the expansion of the materials as the temperature varies,        mechanical displacements resulting from the variations in centrifugal force applied to the rotating parts, and variations in pressure.        
The disks, the blades and the stator elements are subjected to both mechanical and thermal displacements.
During the various engine operating phases, because of these displacements which will not always be in the same direction, the radial clearance is not therefore constant. In particular, the rotor and the stator do not have displacements of equal amplitude, nor do they have the same thermal response time.
FIG. 2 shows the change in displacement of the rotor R and of the stator S respectively as a function of the variation in engine speed over time. Thus, it can be seen that the take-up of transient clearance A is greater than that B obtained after thermal stabilization. The take-up of clearance is to be understood to mean the magnitude of the displacement of the rotor minus that of the stator.